Wetting Transitions on Rough Substrates: General Considerations

General properties of wetting transitions for droplets on rough substrates are analyzed theoretically. The energy barrier to be surpassed for wetting transitions is much lower than the heat of evaporation of the droplet; this makes wetting transitions possible. It is shown that the energy curve of the transition state, i.e., the dependence of the interfacial part of the Gibbs free energy on the apparent contact angle in this state, as well as the energy barriers, can be expressed through the contact angles in the initial and final states without going into the geometric details of the given substrate relief. On this basis, the reason for the irreversibility of the Cassie–Wenzel transitions is elucidated: the energy barrier of the reverse transition is shown to be much higher. The scheme is also applicable to the substrates with disordered reliefs. Time-scaling arguments are important for understanding wetting transitions.     

Time-Dependence of the Adhesion of Micrometer-Size Particles to Substrates: Correlation with Postdeposition Particle Rotation

The time dependence of the detachment force of 7-µm ground polyester particles coated with silica nanoparticles from a ceramer-coated substrate was determined by ultracentrifugation. The detachment force of the particles from the substrate was found to increase with the time since the particle deposition. Scanning electron microscopy (SEM) results show that, following deposition, the particles rotate at approximately the same scale as the observed increase in the detachment force. This suggests that the increase in adhesion may be due to particle rotation from their initial positions obtained upon deposition to a more stable position that results from torques generated by either electrostatic or van der Waals forces acting on the particles.